There are effective behavioral interventions to treat alcohol use disorders, but over time, treatment gains are often lost when clients find themselves unable to resist drinking because of negative emotions and appetitive cues. The power of internal and environmental triggers to elicit relapse, even among clients with conscious abstinence goals and extended abstinence, is compelling and well-documented. Thus, bolstering clients' ability to withstand drinking triggers and craving in real time could have tremendous public health impact. This application proposes translational research that focuses on the a priori defined and malleable baroreflex (BAR) mechanism. The BAR is a dynamic mechanism that helps to regulate automatic-visceral reactivity to triggers of alcohol and other drug use by regulating bidirectional communication between the heart and brain. A key feature of the BAR mechanism is that it can be consciously manipulated using a simple behavioral breathing technique called resonance breathing. This manipulation can occur in the moment and outside of the treatment context as triggers are anticipated or encountered. This application proposes a randomized clinical trial of a BAR-based intervention (added to behavioral treatment as usual) in conjunction with laboratory assessments (pre-post intervention design), and computational modeling to validate the operation of the BAR as a biobehavioral change mechanism. The sample comprises women with young children from an empirically supported, intensive outpatient behavioral treatment program (IOP). The intervention involves daily use of iPhone applications (apps) during IOP treatment weeks 4-12. Those randomized to the active intervention will be trained to use an existing resonance breathing app to activate the BAR mechanism as they anticipate or confront emotions or cues that can trigger relapse. Participants in the placebo group will use an app that does not affect the BAR. Aim 1 will address whether activating the BAR mechanism accelerates and stabilizes positive behavior change, specifically change in alcohol and drug use, anxiety, craving, depression during and after treatment. Aim 2 will compare natural versus manipulated changes in BAR functioning pre- to post- intervention using physiological, and, for a subset of women, fMRI data to correlate biological and behavioral change. Aim 3 will characterize how and for whom the BAR mechanism supports behavior change using computational modeling to capture change across multiple interacting biological systems within a person. The novelty of this study comes from focusing on a well-specified automatic-physiological mechanism, capturing change in the dynamic space of real life replete with triggers and affective changes, characterizing the BAR mechanism across biobehavioral levels using variable-and person-centered quantitative strategies, and focusing on an understudied population whose positive behavior change can have important immediate and long-term health implications. If successful, the proposed research will set the stage for a new generation of mechanism-based intervention approaches and personalized prognostic models.